Process for coating ceramic particles and compositions formed from the same

ABSTRACT

Methods of coating ceramic (e.g., barium titanate-based) particles, as well as articles and compositions formed from the coated particles are provided. The methods involve forming a mixture of barium titanate-based particles and at least two dopant metal solutions. According to some methods, a second solution (e.g., a base) is added to the mixture to cause the dopant metals to sequentially precipitate onto surfaces of the particles. The resulting particles are coated with respective layers having different dopant metal compositions. According to other methods, the mixture of barium titanate-based particles and at least two dopant metal solutions are added to a second solution (e.g., a base) to precipitate the dopant metal or metals to form a coating on surfaces of the barium titanate-based particles. The resulting particles are coated with a homogeneous coating. The coated barium titanate-based particles produced according to the methods of the invention may be further processed to form, for example, green layers and/or dielectric layers in electronic devices such as MLCCs. The methods provide a uniform distribution of the dopant metals throughout the barium titanate-based particulate composition and limit dopant segregation which can improve properties of the dielectric layers amongst other advantages.

FIELD OF THE INVENTION

[0001] The invention relates generally to dielectric materials and, more particularly, to methods of coating barium titanate-based particles and compositions formed from the coated particles.

BACKGROUND OF THE INVENTION

[0002] Barium titanate-based materials, which include barium titanate (BaTiO₃) and its solid solutions, may be used to form dielectric layers in electronic devices such as multilayer ceramic capacitors (MLCCs). Typically, barium titanate-based materials are processed in particulate form and subsequently sintered to form the dielectric material.

[0003] Dopants can be added to barium titanate-based materials to improve properties, in particular electrical properties, of the composition. Typically, the dopants are metallic compounds, often in the form of oxides. In some cases, dopant compounds are added to a barium titanate-based particulate composition in the form of discrete particles. The dopant particles may be mixed with the barium titanate-based particles and, in some cases, further milled to yield the final composition. In other cases, dopant compounds may be coated on surfaces of the barium titanate-based particles to produce the final composition. In either case, the resulting particulate composition may then be dispersed to form a slurry which can be further processed, for example, to form a dielectric layer in an electronic device such as an MLCC.

[0004] The resulting dielectric layers may include regions of dopant segregation as a result of unequal distribution of dopants throughout the particulate composition. This unequal distribution can occur when dopant compound particles are added to barium titanate-based particles, or when dopant compounds precipitate as particles as opposed to on surfaces of barium titanate-based particles in certain coating processes. The dopant segregation can lead to non-uniform properties across the layer and can create defects within the layer that can cause premature device failure, amongst other problems. The problems arising from dopant segregation may be more significant in thinner dielectric layers (e.g., less than 5 microns).

SUMMARY OF THE INVENTION

[0005] The invention provides methods of coating ceramic particles (e.g., barium titanate-based particles) and articles formed from the coated particles.

[0006] In one aspect, a method of coating barium titanate-based particles is provided which includes forming a mixture of barium titanate-based particles and at least two dopant metal solutions. The method further includes sequentially precipitating the dopant metals from the mixture to form a coating on surfaces of the barium titanate-based particles.

[0007] In another aspect, a method of coating barium titanate-based particles is provided which includes forming a mixture of barium titanate-based particles and at least two dopant metal solutions. The method further includes adding the mixture to a second solution to precipitate the dopant metals to form a coating on surfaces of the barium titanate-based particles.

[0008] In another aspect, a method of coating barium titanate-based particles is provided which includes forming a mixture of barium titanate-based particles and at least two dopant metal solutions. The mixture is free of a chelating agent. The method further includes precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles.

[0009] In another aspect, a method of coating barium titanate-based particles is provided which includes forming a mixture of barium titanate-based particles and at least two dopant metal solutions. The mixture is at a first temperature condition. The method further includes precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles while maintaining the mixture within 25° C. of the first temperature condition.

[0010] In another aspect, a green layer that comprises a doped barium titanate-based particulate composition is provided. The green layer has a surface that includes at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron. A multi-layer ceramic capacitor may also be provided that includes at least one dielectric layer formed from the green layer.

[0011] In another aspect, a doped barium titanate-based particulate composition is provided. The composition has a surface including at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron.

[0012] Other aspects, features and advantages will become apparent from the following detailed description and drawings when considered in conjunction with the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 schematically illustrates coated barium titanate-based particles produced according to one set of methods of the present invention.

[0014]FIG. 2 schematically illustrates coated barium titanate-based particles produced according to another set of methods of the present invention.

[0015]FIG. 3 is an EPMA map of the green tape sample produced in Example 1.

[0016]FIG. 4 is an EPMA map of the green tape sample produced in Example 2 according to one method of the present invention.

[0017]FIG. 5 is an EPMA map of the sample produced in Example 3 according to a conventional method.

DETAILED DESCRIPTION

[0018] Methods of coating ceramic (e.g., barium titanate-based) particles and articles formed from the coated particles are provided. The methods involve forming a mixture of the particles and at least two dopant metal solutions. According to some methods, a second solution (e.g., a base) is added to the mixture to cause the dopant metals to sequentially precipitate onto surfaces of the particles. The resulting particles are coated with respective layers having different dopant metal compositions. According to other methods, the mixture of particles and at least two dopant metal solutions are added to a second solution (e.g., a base) to precipitate the dopant metal or metals to form a coating on surfaces of the particles. The resulting particles are coated with a uniform coating. The coated particles produced according to the methods of the invention may be further processed to form, for example, green layers and/or dielectric layers in electronic devices such as MLCCs. The methods can provide a uniform distribution of the dopant metals throughout the particulate composition and limit dopant segregation which can improve properties of the dielectric layers amongst other advantages, as described further below.

[0019] As used herein, “barium titanate-based” compositions refer to barium titanate, solid solutions thereof, or other oxides based on barium and titanium having the general structure ABO₃, where A represents one or more divalent metals such as barium, calcium, lead, strontium, magnesium and zinc and B represents one or more tetravalent metals such as titanium, tin, zirconium and hafnium. One type of barium titanate-based composition has the structure Ba_((1-x))A_(x)Ti_((1-y))B_(y)O₃, where x and y can be in the range of 0 to 1, where A represents one or more divalent metal other than barium such as lead, calcium, strontium, magnesium and zinc and B represents one or more tetravalent metals other than titanium such as tin, zirconium and hafnium. Where the divalent or tetravalent metals are present as impurities, the value of x and y may be small, for example less than 0.1. In other cases, the divalent or tetravalent metals may be introduced at higher levels to provide a significantly identifiable compound such as barium-calcium titanate, barium-strontium titanate, barium titanate-zirconate and the like. In still other cases, where x or y is 1.0, barium or titanium may be completely replaced by the alternative metal of appropriate valence to provide a compound such as lead titanate or barium zirconate. In other cases, the compound may have multiple partial substitutions of barium or titanium. An example of such a multiple partial substituted composition is represented by the structural formula Ba_((1-x-x′-x″))Pb_(x)Ca_(x′)Sr_(x″)O.Ti_((1-y-y′-y″))Sn_(y)Zr_(y′)Hf_(y″)O₂, where x, x′, x″,y, y′, and y″ are each greater than or equal to 0. In many cases, the barium titanate-based material will have a perovskite crystal structure, though in other cases it may not.

[0020] It should be understood that though barium titanate-based particles are preferred, the methods of the invention may utilize other types of ceramic particles.

[0021] The barium titanate-based particles may have a variety of different particle characteristics. The barium titanate-based particles typically has an average primary particle size of less than about 5.0 microns; in some cases, the average primary particle size is less than about 1.0 micron; in some cases, the average primary particle size may be less than about 0.5 micron; in some cases, the average primary particle size is less than about 0.1 micron. In some embodiments, the barium titanate-based primary particles will agglomerate and/or aggregate to form aggregates and/or agglomerates of aggregates. At times, it may be preferable to use barium titanate-based particles in the coating process that are not strongly agglomerated and/or aggregated such that the particles may be relatively easily dispersed, for example, by high shear mixing.

[0022] The barium titanate-based particles may have a variety of shapes which may depend, in part, upon the process used to produce the particles. The barium titanate-based particles may be equiaxed and/or substantially spherical, in particular, if the particles are hydrothermally produced as described further below. When the barium titanate-based particles are milled, they generally have an irregular, non-equiaxed shape.

[0023] The barium titanate-based particles may be produced according to any technique known in the art including hydrothermal processes, solid-state reaction processes, sol-gel processes, as well as precipitation and subsequent calcination processes, such as oxalate-based processes. In some embodiments, it may be preferable to produce the barium titanate-based particles using a hydrothermal process. Hydrothermal processes generally involve mixing a barium source with a titanium source in an aqueous environment to form a hydrothermal reaction mixture which is maintained at an elevated temperature. Barium reacts with titanium to form barium titanate particles which remain dispersed in the aqueous environment to form a slurry. The particles may be washed to remove excess barium ions from the hydrothermal process while being maintained in the slurry. As described further below, the particles in the slurry may be subjected to further processing steps (e.g., dried and/or heat treated) and/or maintained in the slurry until the coating process. When forming barium titanate solid solution particles hydrothermally, sources including the appropriate divalent or tetravalent metal are also added to the hydrothermal reaction mixture. Certain hydrothermal processes may be used to produce substantially spherical barium titanate-based particles having a particle size of less than 1.0 micron and a uniform particle size distribution. Suitable hydrothermal processes for forming barium titanate-based particles have been described, for example, in commonly-owned U.S. Pat. Nos. 4,829,033, 4,832,939, and 4,863,883, which are incorporated herein by reference in their entireties.

[0024] In some embodiments, the barium titanate-based particles may be subjected to a heat treatment step prior to coating. The heat treatment step involves heating the particles, for example, to a temperature between about 700° C. and about 1150° C. to increase average particle size. The increased average particle size can improve the electrical properties (i.e., dielectric constant and dissipation factor) of the heat-treated composition after the coating process, as compared to the compositions that are not heat treated. A suitable heat treatment process is described in commonly-owned, co-pending U.S. patent application Ser. No. 09/689,093, which was filed on Sep. 12, 2000, and is incorporated herein by reference in its entirety. When hydrothermally-produced barium titanate-based particles are subjected to a heat treatment step, the water in the slurry may be removed (e.g., by filtering or decanting) and the particles may be dried at a lower temperature prior to heat treatment.

[0025] As described above, the methods of the present invention involves forming a mixture of at least two dopant metal solutions and barium titanate-based particles. In some cases, the dopant metal solutions are mixed together prior to mixing with the barium titanate-based particles. Pre-mixing the dopant solutions may limit the formation of separate dopant particles in the subsequent coating process which limits dopant segregation and improves the distribution of the dopants throughout the resulting particulate composition, as described further below. However, it should be understood that, in some embodiments, the dopant solutions may be added separately to the barium titanate-based particles to form the mixture. In these embodiments, the dopant metal solutions are added in a manner so that precipitation of one dopant metal does not occur prior to formation of the mixture.

[0026] The dopant solutions may be any solution that includes the dopant metal, typically in ionic form. The solutions may be formed by dissolving a metal salt in a liquid such as water. Suitable metal salts that may be used in the preparation of the dopant solution include, but are not limited to, metal nitrates, acetates, chlorides, ammoniated salts, sulfates, and citrates.

[0027] The dopant metals are selected to impart the resulting composition with the desired properties (e.g., electrical properties such as dielectric constant and dissipation factor). Any dopant metal known in the art may be used including Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, Ce, Nb, Bi, Co, Ta, Zn, Al, Ca, Nd, and Sm. For some MLCC applications, Y, Mg and Mn may be preferred dopant metals. The resulting composition may include any number of different dopant metals. Thus, to produce the resulting composition, any number of dopant solutions may be mixed together. For example, in some methods, at least four dopant solutions are mixed together.

[0028] In some methods of the invention, the concentration of the dopant metal solutions are relatively low. For example, in some cases, each dopant metal solutions have a concentration of less than about 0.5 moles metal salt/kg solvent; in other cases, less than about 0.25 moles metal salt/kg solvent; and, in other cases, less than about 0.10 moles metal salt/kg solvent. As described further below, using low dopant metal solution concentrations may advantageously limit the amount of dopant segregation in the resulting composition.

[0029] In some cases, the dopant solutions may be mixed with other components prior to mixing with the barium titanate-based particles. For example, the dopant metal solutions may be mixed with an acid, such as HNO₃, to decrease the pH to a value of less than 7, such as about 3 or about 4.

[0030] To form the mixture of barium titanate-based particles and at least two dopant metal solutions, it is generally preferable to add the particles to the dopant metal solutions. This can avoid uncontrolled, premature precipitation of the dopants which can occur if the dopant metal solution mixture is added to the particles. However, it should be understood that the dopant metal solution mixture may be added to the particles if premature precipitation of the dopants can otherwise be controlled.

[0031] The barium titanate-based particles are typically dispersed in a liquid, for example to form a slurry, prior to being added to the mixture of dopant metal solutions (or prior to having the dopant metal solution mixture being added thereto). The liquid is generally aqueous-based having water as its primary component. However, in some cases, the liquid may be non-aqueous-based (e.g., an organic liquid). Other components may also be present in the liquid including ionic species.

[0032] If the barium titanate-based particles are hydrothermally produced, then the resulting slurry which includes the particles may be directly added to the mixture of dopant metal solutions. Thus, in these cases, the particles do not have to be separately dried and/or redispersed prior to being added to the dopant metal solutions. If the barium titanate-based particles are produced using non-hydrothermal techniques and/or subjected to a heat treatment step, then it may be desirable to disperse the particles in a liquid prior to adding to the dopant metal solutions. It should also be understood that, in some cases, dried and/or heat-treated barium titanate-based particles may be added directly to the dopant metal solutions without being separately dispersed in a liquid.

[0033] The mixture of barium titanate-based particles and at least two dopant metal solutions generally includes between about 5 and about 50 weight percent particles based on the total weight of the mixture; in some cases, between about 10 and about 30 weight percent based particles based on the total weight of the slurry are present. However, it should be understood that weight percentages outside of these ranges can also be used in certain processes.

[0034] The mixture of barium titanate-based particles and at least two dopant metal solutions generally has a pH of less than about 7. In some cases, the pH of the mixture is between about 4 and about 5.

[0035] The mixture of barium titanate-based particles and at least two dopant metal solutions is typically maintained in a processing tank. Within the tank, the mixture is mixed (i.e., stirred), for example using a high shear mixer, so as to ensure a relatively homogeneous dispersion of particles throughout the mixture and to prevent settling of the particles. Suitable high shear mixers are commercially available including Silverson L4RT (manufactured by Silverson Machines Incorporated, East Longmeadow, Mass.) and Ross Models ME-105 and ME-110 (manufactured by Charles Ross and Son Company, Hauppauge, N.Y.).

[0036] According to some methods of the present invention, a second solution is added to the mixture of barium titanate-based particles and at least two dopant metal solutions. The second solution is typically a base which is added in a manner that gradually increases the pH of the mixture. In some embodiments, a metal hydroxide, such as barium hydroxide, is the preferred base. Because different dopant metals precipitate at different pH values, the dopant metals sequential precipitate on surfaces of the barium titanate-based particles as the pH is increased. The dopants metals typically precipitate as metal oxides or hydroxides. Table 1 shows the pH at which different common dopant metal compounds precipitate (data from Marcel Porbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, Tex., 1974). As indicated on the table, in cases when barium ions are present, certain dopant metals (e.g., Si, W, Mo, V, and Cr) precipitate as complex oxides. Dopant Precipitation Compound pH Y(OH)₃ 6.62 Ho(OH)₃ 6.45 Mg(OH)₂ 8.48 Mn(OH)₂ 7.66 BaSiO₃ 11.84 BaWO₄ 7.03 BaMoO₄ 5.38 Ba₂V₂O₇ 9.15 BaCr₈O₁₃ 4.6

[0037] The rate at which the second solution is added depends on a variety of processing parameters including the volume of the particle and dopant metal solution mixture (i.e., batch size), the total weight of the particles, particle size, and the type and concentration of the second solution, amongst others. For a 40 gallon batch size that includes 25 kg of barium titanate-based particles having a particle size of about 0.4 micron, a suitable rate for the introduction of 0.2 moles Ba(OH)₂/kg solvent into the mixture is about 2.5 kg/min. It should be understood that other rates would also be effective.

[0038] Preferably, the second solution is added while the mixture is being mixed (i.e., stirred) to promote a substantially constant pH throughout the mixture. This can prevent precipitation of the dopant metals on particle surfaces or as separate particles in localized areas of the mixture which may result in an inhomogeneous distribution of dopants in the final composition. The second solution may be introduced into the mixture, for example, through the nozzle of a high shear mixture. In some cases, it may be preferable to introduce the second solution in an area of high shear to optimize mixture uniformity.

[0039] The second solution may be at an elevated temperature when it is added to the mixture. For example, when the second solution is Ba(OH)₂, it may be preferable for the second solution to be at a temperature between about 60° C. and about 90° C. (e.g., 80° C.) to increase the solubility of barium in the solution.

[0040] The pH of the mixture is generally increased to a value above the pH required to cause all of the dopant metals in the mixture to precipitate on surfaces of the barium titanate-based particles. The specific value to which the pH is increased, therefore, depends on the types of dopant metals in solution. In some cases, the pH of the mixture is increased from a value of between about 4 and about 5 to a value of between about 7 and about 12.

[0041] The dopants precipitate in the form of dopant metal compounds, as described above. The sequential precipitation of the dopant metal compounds on surfaces of the barium titanate-based particles leads to particle coatings that have a series of chemically distinct regions or layers. FIG. 1 schematically illustrates barium titanate-based particles 10 that include a coating 12 formed by the sequential precipitation method of the invention. Although the figure shows coatings that encompass the underlying particle, it is understood that the precipitation of the dopant compounds can result in regions or layers that cover only a portion or portions of the particle, and successively deposited dopants may not cover, or may only partially cover, a previously deposited dopant layer. Coating 12 has a first layer 14 that is deposited on particles 10 and is formed substantially of a first dopant metal compound. A second layer 16 of the coating is deposited on the first layer and is formed substantially of a second dopant metal compound. A third layer 18 of the coating is deposited on the second layer and is formed substantially of a third dopant metal compound. A fourth layer 20 of the coating is deposited on the third layer and is formed substantially of a fourth dopant metal compound. Subsequent layers may be formed in a similar fashion, though not illustrated.

[0042] Following the process described above, it should be understood that first layer 14 is precipitated at a lower pH than second layer 16, which is precipitated at a lower pH than third layer 18, which is precipitated at a lower pH than fourth layer 20. In one embodiment, the particles include a coating having a first layer formed of Y(OH)₃, a second layer formed of Mn(OH)₂, a third layer formed of Mg(OH)₂, and a fourth layer formed of BaSiO₃.

[0043] It should be understood that the respective layers of the coating may not be entirely chemically homogeneous. That is, there may be a small percentage of other dopant metal compounds within each layer and, in particular, near interfaces between adjacent layers. For example, second layer 16 may include a small percentage of one of the other dopant metal compounds, such as the first dopant metal compound or the third dopant metal compound. The first dopant metal compound would most likely be present within the second layer near the interface between the first and second layer, while the third metal compound would most likely be present near the interface between the second and the third layer. These small amounts of inhomogeneity within the layers do not significantly effect the overall uniformity of the composition.

[0044] In other methods of the present invention, a mixture of barium titanate-based particles and at least two dopant metal solutions is added to a second solution. These methods of the invention differ, at least in some respects as described further below, from methods of the invention described above in which the second solution is added to the mixture of barium titanate-based particles and at least two dopant metal solutions.

[0045] In these methods, the mixture of barium titanate-based particles and at least two dopant metal solutions are typically added to a base. In some embodiments, a metal hydroxide, such as barium hydroxide, is the preferred base. The addition of the particle and dopant metal solution mixture to the base causes the pH of the particle metal solution mixture to rapidly increase. This rapid increase in the pH is to be contrasted with the relatively gradual increase in pH of the mixture in methods described above in which the base is added to the mixture. The increase in the pH of the particle and dopant metal solution mixture is typically coupled with a decrease in pH of the second solution. For example, the pH of the second solution may decrease from a pH of about 13 to a pH of about 12 after the addition of the particle and dopant metal solution mixture.

[0046] Generally, the pH of the mixture is rapidly increased to a value above the pH required to cause all of the dopant metals in the mixture to precipitate on surfaces of the barium titanate-based particles. The specific value to which the pH is increased, therefore, depends on the types of dopant metals in solution. In some cases, the pH of the mixture is increased from a value of between about 4 and about 5 to a value of between about 7 and about 12 after being added to the base.

[0047] However, it should be understood that, in certain embodiments of the invention, the pH of the mixture may be rapidly increased to a value sufficient to cause at least two of the dopant metals to precipitate on surfaces of the particles, but insufficient to cause all of the dopant metals to precipitate. In these embodiments, the pH of the mixture may be increased more gradually to precipitate the remaining dopant metals in the mixture.

[0048] The mixture of barium titanate-based particles at least two dopant metal solutions should be added to the second solution in a manner that enables the pH of the particle metal solution to be increased rapidly. The rate may depend on a variety of processing parameters including the volume, type and concentration of the second solution, the batch size, the total weight of the particles, particle size, amongst others.

[0049] In order to form a 40 gallon batch mixture that includes 25 gallons of 0.2 moles Ba(OH)₂/kg solvent and barium titanate-based particles having a particle size of about 0.4 micron, a suitable rate for the introduction of the particle and dopant metal solution mixture is about 10.0 kg/min. It should be understood that other rates would also be effective.

[0050] Preferably, the mixture of barium titanate-based particles and at least two dopant metal solutions is added while the second solution is being mixed (i.e., stirred) to promote a substantially constant pH throughout the mixture. This can prevent precipitation of the dopant metals in localized areas of the mixture which may result in an inhomogeneous distribution of dopants in the final composition. The mixture may be introduced into the second solution, for example, through the nozzle of a high shear mixture. In some cases, it may be preferable to introduce the mixture of barium titanate-based particles and at least two dopant metal solutions in an area of high shear to optimize mixture uniformity.

[0051] The second solution to which the mixture of barium titanate-based particles and at least two dopant metal solutions is added may be maintained at an elevated temperature in some cases. For example, when the second solution is Ba(OH)₂, it may be preferable for the second solution to be maintained at a temperature between about 60° C. and about 90° C. (e.g., 80° C.) to increase the solubility of the barium in the solution.

[0052] Due to the rapid increase in pH resulting from the mixture being added to base, different dopant metal compounds precipitate on surfaces of the barium titanate-based particles at approximately the same time. For example, Y(OH)₃ (precipitates at pH 6.62), BaMoO₄ (precipitates at pH 5.38) and Mg(OH)₂ (precipitates at 8.48) each precipitate on particle surfaces at relatively the same time. Thus, the coatings formed on the barium titanate-based particles are relatively homogeneous with each dopant distributed relatively uniformly throughout the coating. FIG. 2 schematically illustrates barium titanate-based particles 10 that include a homogeneous coating 22 formed according to these methods of the invention. As described above, the dopant metal compounds which form the coating layer are typically metal oxides or hydroxides. Examples of typical dopant metal compounds are shown in Table 1.

[0053] It should be understood that the homogeneous coatings may not include a perfectly homogeneous distribution of dopants. For example, there may be a slightly greater concentration of dopants (e.g., Cr) that precipitate at a low pH proximate the particle/coating interface than at a greater distance from the particle; and, a slightly greater concentration of dopants (e.g., Mg) that precipitate at a higher pH at a greater distance from the particle than proximate the particle/coating interface. These small amounts of non-uniformity within the coating do not significantly effect the overall uniformity of the composition.

[0054] The different coating methods described above may provide coatings having a similar overall thickness. The coating thickness depends in part upon the desired weight percentage of the dopant and the size of the barium titanate-based particle. The overall thickness of the dopant coating, for example, may be between about 0.1 nm and about 10.0 nm; in some cases, the thickness may be between about 0.5 nm and about 5.0 nm. In embodiments in which coatings having respective chemically distinct layers are formed, the individual layers have thicknesses that are a fraction of the overall thickness of the dopant coating.

[0055] The different coating methods described above may provide coatings over the entire particle surface. In some embodiments, the coating may have a uniform thickness such that the thickness of the coating varies by less than 20%. In other cases, the thickness may vary by larger amounts across the surface of an individual particle. Particularly in cases when the dopant percentage is low (i.e. less than 0.5 weight percent), the thickness of the coating may vary over different portions of the particles and, sometimes, portions of the particle may not be coated. Also, when particles are irregularly shaped, for example due to aggregation and/or agglomeration, the thickness may vary over different portions of the particles. Some particles of the barium titanate-based composition may not be coated at all.

[0056] The weight percentage of the dopant present may be selected to provide the composition with the desired electrical properties and is generally irrespective of the coating method used. Generally, the barium titanate-based composition includes less than about 5 weight percent of each individual dopant element based upon the total weight of the barium titanate-based particulate composition. For example, in some cases, each individual dopant element weight percentage is between about 0.0020 and about 1.0 based upon the total weight of the barium titanate-based particulate composition; and, in some cases, each individual dopant element weight percentage is between about 0.0025 and about 0.1 based upon the total weight of the barium titanate-based particulate composition. In some cases, the total weight percentage of all dopants in the composition is between about 0.05 weight percent and about 10 weight percent based on the total weight of composition; and in some cases, between about 0.1 weight percent and about 5 weight percent.

[0057] Advantageously, the methods of the present invention are relatively simple and can be performed using a relatively small number of different chemical compounds and a relatively few number of processing steps. For example, the methods do not require use of a separate chelating agent, in contrast with certain prior art coating techniques. Thus, in some embodiments, the methods of the present invention form a mixture of barium titanate-based particles and at least two dopant metal solutions which is free of a chelating agent. Furthermore, the methods do not require a heating step to promote precipitation of the dopant metals on particle surfaces, in contrast with certain prior art coating techniques. For example, in some embodiments, the mixture of barium titanate-based particles and at least two dopant metal solutions is at a first temperature condition and the precipitation of the dopant metals to form a coating on surfaces of the particles occurs at a temperature within 25° C. of the first temperature condition. This is generally achievable even in methods of the present invention that may pre-heat the second solution to an elevated temperature (e.g., between about 60° C. and about 90° C.) as described above. In some embodiments, the precipitation occurs at a temperature within 10° C. of the first temperature condition, or even at about the same temperature as the first temperature condition.

[0058] The coated particles produced according to any of the above-described coating methods are typically further processed to achieve the desired final product. Immediately after coating the particles remain dispersed in a liquid, typically as a slurry. In some methods, the particles are filtered and washed, for example, using buffered water. Additives such as dispersants and/or binders may be added to the slurry to form a castable slip as described further below. In some embodiments, a portion of the aqueous phase may be eliminated from the slurry to form a wet cake. In other embodiments, the coated barium titanate-based particles may be recovered from the slurry and dried. In some embodiments, other particulate compounds may be added to the composition, for example, to adjust the A/B ratio of the composition. Ultimately, the barium titanate-based particles may be processed to form dielectric layers in electronic applications such as MLCCs. During the formation of dielectric layers, the particulate composition is typically cast to form a green layer, for example a green tape, which is sintered to form a densified dielectric layer. The green layer may include additives such as a binder or a dispersant which are added to facilitate casting.

[0059] The methods of the invention produce barium titanate-based compositions that have a uniform distribution of dopant metals throughout the composition. The uniform distribution results from the dopants being coated on surfaces of the barium titanate-based particles, as opposed to being present as separate dopant particles. The coating methods of the invention limits, or eliminates, the formation of even a small number of dopant particles which may occur in certain prior art coating processes. Dopant particle formation is limited, or eliminated, by reducing the tendency of such particles to nucleate. In some cases, dopant particle nucleation is limited by using relatively dilute dopant metal solutions (e.g., less than 0.5 moles salt/kg solvent) and/or by mixing to ensure that the mixture includes a homogeneous concentration of dopant metal solutions throughout. As a result of the uniform dopant distribution, the methods limit, or eliminate, the presence of regions of dopant segregation within green layers (i.e., non-sintered layers) and/or sintered dielectric layers formed using the compositions. The absence or limited amount of dopant segregation in green layers or dielectric layers formed using compositions produced according to the methods of the present invention results in longer lifetimes and improved performance for MLCCs which include such layers.

[0060] The amount of dopant segregation in a green layer formed from barium titanate-based compositions may be characterized, for example, using Electron Probe Microscopy Analysis (EPMA). EPMA is a known technique that can generate a compositional map of an indicative area(s) on the surface of a composition, or a green layer formed from the composition. The size of dopant segregates, which may be a single dopant particle or multiple dopant particles held together, can be determined from the EPMA maps. In some cases, the composition or a green layer formed from the compositions includes at least one 100 micron by 100 micron area that includes less than twenty five dopant segregates having a size of greater than about 2.0 micron; in other cases, the composition or a green layer formed from the compositions includes at least one 100 micron by 100 micron area that includes less than ten, or less than five, dopant segregates having a size of greater than about 2.0 micron. In some cases, the composition or a green layer formed from the compositions includes at least one 100 micron by 100 micron area, or 1 cm by 1 cm area, that is substantially free of dopant segregates having a size of greater than about 2.0 micron.

[0061] The majority of the surface area of the barium titanate-based compositions or green layers produced using the compositions may be substantially free of dopant segregates. For example, if the entire composition or layer surface is divided into 100 micron by 100 micron areas, then, in some cases, greater than 50% of such areas are substantially free of dopant segregates having a size of greater than about 1.0 micron; in other cases, at least about 75% of such areas are substantially free of dopant segregation; and, in other cases, at least about 95% of such areas are substantially free of dopant segregation.

[0062] In some cases, the green layers having the above-identified characteristics are green tapes. In some cases, MLCCs are formed from green layers having the above-identified characteristics. In some cases, sintered dielectric layers in the MLCCs include the same above-identified characteristics. As used herein, the term “substantially free” refers to areas that have one or less dopant segregates of greater than a characteristic size (e.g., about 2 micron). In some cases, “substantially free” may refer to compositions that have no dopant segregates of greater than the characteristic size (e.g., about 2 micron).

[0063] It should be understood that the above-described advantages and characteristics associated with lack of dopant segregation may be present in compositions and articles formed therefrom that include particles having a series of chemically distinct layers (as shown in FIG. 1), as well as compositions and articles formed therefrom that include particles having a chemically homogeneous coating (as shown in FIG. 2).

[0064] The present invention will be further illustrated by the following examples, which are intended to be illustrative in nature and are not to be considered as limiting the scope of the invention.

EXAMPLE 1

[0065] This example involves the production of a coated barium titanate-based particulate composition according to a method of the present invention and the characterization of a green layer made from the composition.

[0066] Barium titanate-based particles having an average particle size of about 0.2 micron were formed in a hydrothermal process. The particles were subjected to a heat treatment step which involved heating the particles to about 1000° C. for about 3 hours. After heat treatment, the particles had an average particle size of about 0.4 micron. The particles were dispersed in water to form an aqueous slurry that included about 35 percent by weight of the particles.

[0067] A solution of 0.1 mol Ho(NO₃)₃/kg water was added to a tank followed by the addition of a solution of 0.1 mol Mn(NO₃)₂/kg water and the addition of a solution of 0.1 mol Mg(NO₃)₂/kg water. In a separate container, HNO₃ was added to a solution of 0.1 mol NaSiO₃/kg water to adjust the pH to about 7. Then, the solution was added to the tank with the other dopant metal solutions. The pH of the mixture of dopant metal solutions in the tank was adjusted to about 3 by adding HNO₃.

[0068] The slurry of barium titanate-based particles was added to the dopant solution mixture to form a mixture of barium titanate-based particles and dopant metal solutions. During and after the addition of the particles, a Silverson L4RT high shear mixer agitated the mixture at 8000 RPM.

[0069] A 0.2 mole/kg Ba(OH)₂.8H₂O was added was added dropwise to the mixture of barium titanate-based particles and dopant solutions through the nozzle of the high shear mixer until the pH reached 12.5. The mixer continued to agitate the mixture at 8000 RPM.

[0070] The dopant metals successively precipitated out onto surfaces of the barium titanate-based particles to form a series of chemically distinct layers. In succession from the particle surface, the layers were formed of Ho(OH)₃, Mn(OH)₂, Mg(OH)₂, and BaSiO₃.

[0071] The resulting particulate slurry was filtered and washed to form a wet cake. The filtering and washing steps were repeated until the filtrate conductivity was below 500 μS. A dispersant (Darvan 821A manufactured by RT Vanderbilt Company, Murray, Ky.) and a polymeric binder were added to the wet cake to form a castable slip. The slip was cast to form a green tape.

[0072] The green tape was characterized using an electron probe microanalysis (EPMA) technique. The technique generated a compositional map of a 100 micron by 100 micron area on the surface of the tape. Dopant segregates are defined as areas on the map of a dopant metal that are greater than 1 micron in diameter. FIG. 3 is an example of a typical EPMA map of the tape sample. FIG. 3 shows that the samples include no dopant segregates having a size of greater than 1 micron within the map area.

[0073] This example establishes that methods of the present invention may be used to form a green layer that is substantially free of dopant segregation.

EXAMPLE 2

[0074] This example involves the production of a coated barium titanate-based particulate composition according to a method of the present invention and the characterization of a green layer made from the composition.

[0075] A mixture of barium titanate-based particles and dopant solutions was produced as described in Example 1. The mixture of barium titanate-based particles and dopant solutions was added to a 0.2 mole/kg Ba(OH)₂.8H₂O solution. The mixture was introduced through the nozzle of a Silverson L4RT high shear mixer, while the solution was being agitated at 8000 RPM.

[0076] The dopant metals simultaneously precipitated out onto surfaces of the particles to form a coating layer having the dopant metal compounds (Ho(OH)₃, Mn(OH)₂, Mg(OH)₂, and BaSiO₃) distributed uniformly throughout. The resulting particulate slurry was filtered and washed to form a wet cake. The filtering and washing steps were repeated until the filtrate conductivity was below 500 μS.

[0077] Tape samples were produced and characterized using the EPMA technique as described above. FIG. 4 is an example of a typical EPMA map of the tape sample. FIG. 4 shows that the samples include no dopant segregates having a size of greater than 1 micron within the map area.

[0078] This example establishes that methods of the present invention may be used to form a green layer that is substantially free of dopant segregation.

EXAMPLE 3

[0079] This example involves the characterization of a green layer formed from a conventional barium titanate-based composition.

[0080] A tape sample was produced using a conventional barium titanate-based particulate composition that includes dopant particles. The concentration of dopant particles within the composition was similar to the dopant concentration of the compositions produced in Examples 1 and 2. A green tape sample was analyzed using the EPMA technique as described above. FIG. 5 is an example of a typical EPMA map of the tape sample. FIG. 5 shows the presence of a large number (over 50) of dopant segregates having a size of greater than 1 micron.

[0081] This example establishes the presence of dopant segregation in a conventional barium titanate-based particle composition and green layers made from the composition.

[0082] It should be understood that although particular embodiments and examples of the invention have been described in detail for purposes of illustration, various changes and modifications may be made without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited except as by the appended claims. 

What is claimed is:
 1. A method of coating barium titanate-based particles comprising: forming a mixture of barium titanate-based particles and at least two dopant metal solutions; and sequentially precipitating the dopant metals from the mixture to form a coating on surfaces of the barium titanate-based particles.
 2. The method of claim 1, wherein the pH of the mixture is increased to sequentially precipitate each of the dopant metals.
 3. The method of claim 2, wherein the pH of the mixture is increased by adding a base to the mixture.
 4. The method of claim 3, wherein the base comprises barium hydroxide.
 5. The method of claim 3, further comprising mixing the mixture while adding a base to the mixture.
 6. The method of claim 5, wherein mixing the mixture creates a region of high shear and the base is introduced into the region of high shear.
 7. The method of claim 2, wherein a first dopant metal is precipitated at a first pH and a second dopant metal is precipitated at a second pH greater than the first pH.
 8. The method of claim 2, wherein a first dopant metal is precipitated at a first pH, a second dopant metal is precipitated at a second pH greater than the first pH, a third dopant metal is precipitated at a third pH greater than the second pH, and a fourth dopant metal is precipitated at a fourth pH greater than the third pH.
 9. The method of claim 2, wherein the pH of the mixture is increased from a value of between about 4 and about 5 to a value of between about 7 and about
 12. 10. The method of claim 1, further comprising mixing at least two dopant metal solutions and adding the barium titanate-based particles to form the mixture of barium titanate-based particles and at least two dopant metal solutions.
 11. The method of claim 10, wherein the at least two dopant metal solutions are mixed to form a solution having a pH of less than about
 4. 12. The method of claim 1, wherein the dopant metal solutions comprise a metal selected from the group consisting of Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, and Ce.
 13. The method of claim 12, wherein the dopant metal solutions comprise a metal selected from the group consisting of Y, Mg, and Mn.
 14. The method of claim 1, wherein the mixture comprises at least four dopant metal solutions.
 15. The method of claim 1, wherein the coating includes a plurality of layers, each layer comprising a different dopant metal.
 16. The method of claim 1, wherein the coating comprises a dopant metal oxide or metal hydroxide.
 17. The method of claim 1, further comprising forming the barium titanate-based particles in a hydrothermal process.
 18. The method of claim 17, further comprising maintaining the barium titanate-based particles in an aqueous slurry after the hydrothermal process and adding the aqueous slurry of barium titanate-based particles to the at least two dopant metal solutions.
 19. The method of claim 1, further comprising processing the coated barium titanate-based particles to form a dielectric layer in a multi-layer ceramic capacitor.
 20. The method of claim 1, wherein the dopant metal solutions are aqueous solutions.
 21. The method of claim 1, wherein forming the mixture of barium titanate-based particles and the at least two dopant metal solutions comprises adding barium titanate-based particles to at least one metal solution to form a first mixture and adding the first mixture to a second dopant metal solution to form the mixture of barium titanate-based particles and at least two dopant metal solutions.
 22. A method of coating barium titanate-based particles comprising: forming a mixture of barium titanate-based particles and at least two dopant metal solutions; and adding the mixture to a second solution to precipitate the dopant metals to form a coating on surfaces of the barium titanate-based particles.
 23. The method of claim 22, further comprising mixing at least two dopant metal solutions and adding the barium titanate-based particles to form the mixture of barium titanate-based particles and at least two dopant metal solutions.
 24. The method of claim 22, wherein the at least two dopant metal solutions are mixed to form a solution having a pH of less than about
 4. 25. The method of claim 22, wherein the mixture includes at least four dopant metal solutions.
 26. The method of claim 22, wherein the dopant metal solutions comprise a metal selected from the group consisting of Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, and Ce.
 27. The method of claim 26, wherein the dopant metal solutions comprise a metal selected from the group consisting of Y, Mg, and Mn.
 28. The method of claim 22, wherein the second solution is a basic solution.
 29. The method of claim 28, wherein the basic solution comprises barium hydroxide.
 30. The method of claim 22, wherein the second solution increases the pH of the mixture to a value of between about 7 and about
 12. 31. The method of claim 22, further comprising mixing the solution while adding the mixture of barium titanate-based particles and at least two dopant metal solutions to the solution.
 32. The method of claim 31, wherein mixing the solution creates a region of high shear and the mixture of barium titanate-based particles and at least two dopant metal solutions is introduced into the region of high shear.
 33. The method of claim 22, wherein each dopant metal is distributed throughout the coating.
 34. The method of claim 22, wherein the coating comprises a dopant metal oxide or metal hydroxide.
 35. The method of claim 22, further comprising forming the barium titanate-based particles in a hydrothermal process.
 36. The method of claim 35, further comprising maintaining the barium titanate-based particles in an aqueous slurry after the hydrothermal process and adding the aqueous slurry of barium titanate-based particles to the at least two dopant metal solutions.
 37. The method of claim 22, further comprising processing the coated barium titanate-based particles to form a dielectric layer in a multi-layer ceramic capacitor.
 38. The method of claim 22, wherein the dopant metal solutions are aqueous solutions.
 39. The method of claim 22, wherein forming the mixture of barium titanate-based particles and the at least two dopant metal solutions comprises adding barium titanate-based particles to at least one metal solution to form a first mixture and adding the first mixture to a second dopant metal solution to form the mixture of barium titanate-based particles and at least two dopant metal solutions.
 40. A method of coating barium titanate-based particles comprising: forming a mixture of barium titanate-based particles and at least two dopant metal solutions, the mixture being free of a chelating agent; and precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles.
 41. The method of claim 40, wherein the pH of the mixture is increased to precipitate each of the dopant metals.
 42. A method of coating barium titanate-based particles comprising: forming a mixture of barium titanate-based particles and at least two dopant metal solutions, the mixture being at a first temperature condition; and precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles while maintaining the mixture within 25° C. of the first temperature condition.
 43. The method of claim 42, wherein the pH of the mixture is increased to precipitate each of the dopant metals.
 44. A green layer comprising a doped barium titanate-based particulate composition and having a surface that includes at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron.
 45. The green layer of claim 44, wherein the doped barium titanate-based particulate composition includes barium titanate-based particles that include a coating comprising at least two dopant metals.
 46. The green layer of claim 45, wherein the coating comprises at least two dopant metal oxides or metal hydroxides.
 47. The green layer of claim 45, wherein the coating includes a plurality of layers, each layer comprising a different dopant metal.
 48. The green layer of claim 45, wherein each dopant metal is distributed throughout the coating.
 49. The green layer of claim 44, wherein the barium titanate-based particles are formed in a hydrothermal process.
 50. The green layer of claim 44, wherein the barium titanate-based particles are substantially spherical.
 51. The green layer of claim 44, wherein the green layer includes at least one 1 cm by 1 cm micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron.
 52. The green layer of claim 44, wherein the total weight percentage of dopants in the composition is between about 0.05 weight percent and about 10 weight percent based on the total weight of composition.
 53. The green layer of claim 44, further comprising a polymeric binder.
 54. The green layer of claim 44, wherein the green layer is a green tape.
 55. The green layer of claim 44, wherein the surface includes at least one 100 micron by 100 micron area that includes less than about ten dopant segregates having a size of greater than about 2.0 micron
 56. The green layer of claim 55, wherein the surface includes at least one 100 micron by 100 micron area that includes less than about five dopant segregates having a size of greater than about 2.0 micron
 57. The green layer of claim 56, wherein the surface includes at least one 100 micron by 100 micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron.
 58. A multi-layer ceramic capacitor including at least one dielectric layer formed from the green layer of claim
 44. 59. A doped barium titanate-based particulate composition having a surface including at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron.
 60. The doped barium titanate-based particulate composition of claim 59, wherein the surface includes at least one 100 micron by 100 micron area that includes less than about ten dopant segregates having a size of greater than about 2.0 micron.
 61. The doped barium titanate-based particulate composition of claim 60, wherein the surface includes at least one 100 micron by 100 micron area that includes less than about five dopant segregates having a size of greater than about 2.0 micron.
 62. The doped barium titanate-based particulate composition of claim 61, wherein the surface includes at least one 100 micron by 100 micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron. 